Terpene compounds represent a wide range of natural molecules with a large diversity of structure. The plant kingdom contains the highest diversity of monoterpenes and sesquiterpenes. Often they play a role in defense of the plants against pathogens, insects and herbivored and for attraction of pollinating insect.
The biosynthesis of terpenes has been extensively studied in many organisms. The common precursor to terpenes is isopentenyl pyrophosphate (IPP) and many of the enzymes catalyzing the steps leading to IPP have been characterized. Two distinct pathways for IPP biosynthesis are currently known (FIG. 1). The mevalonate pathway is found in the plants cytosol and in yeast and the non-mevalonate pathway (or deoxyxylulose-5-phosphate (DXP) pathway is found in the plant plastids and in E. coli. 
For example, the IPP is isomerized to dimethylallyl diphosphate by the IPP isomerase and these two C5 compounds can be condensed by prenyl transferases to form the acyclic pyrophosphate terpene precursors for each class of terpenes, i.e. geranyl-pyrophosphate (GPP) for the monoterpenes, farnesyl-pyrophosphate (FPP) for the sesquiterpenes, geranylgeranyl-pyrophosphate (GGPP) for the diterpenes (FIG. 2). The enzymes catalyzing the cyclisation step of the acyclic precursors are named terpene cyclases or terpene synthases, which are referred to as terpene synthases herein.
These enzymes may be able to catalyze complex multiple step cyclization to form the carbon skeleton of a terpene or sesquiterpene compound. For example, the initial step of the catalyzed cyclisation may be the ionization of the diphosphate group to form a allylic cation. The substrate then undergoes isomerizations and rearrangements which can be controlled by the enzyme active site. The product may, for example, be acyclic, mono-, di or tri-cyclic. A proton may then be released from the carbocation or the carbocation reacts with a water molecule and the terpene hydrocarbon or alcohol is released. Some terpene synthases produce a single product, but many produce multiple products.
A large diversity of terpene structures and sesquiterpene synthases are found in nature. Several sesquiterpene synthase encoding cDNA or genes have been cloned and characterized from different plant sources, e.g, 5-epi-aristolochene synthases form Nicotiana tabacum (Facchini, P. J. and Chappell, J. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11088-11092.) and from Capsicum annum (Back, K., et al. (1998) Plant Cell Physiol. 39 (9), 899-904.), a vetispiradiene synthase from Hyoscyamus muticus (Back, K. and Chappell, J. (1995) J. Biol. Chem. 270 (13), 7375-7381.), a (E)-β-farnesene synthase from Mentha piperita (Crock, J., et al. (1997) Proc. Natl. Acad. Sci. U.S.A. 94 (24), 12833-12838.), a δ-selinene synthase and a γ-humulene synthase from Abies grandis (Steele, C. L., et al. (1998) J. Biol. Chem. 273 (4), 2078-2089.), δ-cadinene synthases from Gossypium arboreum (Chen, X. Y., et al. (1995) Arch. Biochem. Biophys. 324 (2), 255-266; Chen, X. Y., et al. (1996) J. Nat Prod. 59, 944-951.), a E-α-bisabolene synthase from Abies grandis (Bohlmann, J., et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95 (12), 6756-6761.), a germacrene C synthase from Lycopersicon esculentum (Colby, S. M., et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95 (5), 2216-2221.), an epi-cedrol synthase and an amorpha-4,11-diene synthase from Artemisia annua (Mercke, P., et al. (1999) Arch. Biochem. Biophys. 369 (2), 213-222; Mercke, P., et al. (2000) Arch. Biochem. Biophys. 381 (2), 173-180.), and germacrene A synthases from Lactuca sativa, from Cichorium intybus and from Solidago canadensis (Bennett, M. H., et al. (2002) Phytochem. 60, 255-261; Bouwmeester, H. J., et al. (2002) Plant Physiol. 129 (1), 134-144; Prosser I, et al. (2002) Phytochem. 60, 691-702).
Many sesquiterpene compounds are used in perfumery (e.g. patchoulol, nootkatone, santalol, vetivone, sinensal) and many are extracted from plants. As a result, their availability and prices may be subject to fluctuation related to the availability of the plants and the stability of the producing countries. The availability of a plant-independent system for production of sesquiterpenes may therefore be of interest. Due to the structural complexity of some sesquiterpenes, their chemical synthesis at an acceptable cost may not always be feasible.